The present invention is generally directed to an improved method and apparatus for generating acid and base solutions by electrodialysis of a salt solution and more particularly to an improved method and apparatus for the in-situ regeneration of exhausted ion-exchange resins.
Water treatment systems containing anion and cation exchange resins have many industrial applications. A primary application of such systems is in the purification of water for condensate recirculation systems used to drive steam turbines. During the service cycle of such systems the ion exchange resins become exhausted and must be regenerated prior to return for use in a subsequent service cycle. It is conventional to regenerate the cation exchange resin with a strong acid, such as hydrochloric acid, and to regenerate the anion exchange resin with a strong base, such as sodium hydroxide. It is the customary practice to purchase and store large quantities of regenerant acid and base for use in such systems.
It is well known in the art that acid and base solutions may be generated from a salt solution by either an electrolysis process or by an electrodialysis water splitting process. In the electrolysis process the generation of acid and base solutions is achieved through the use of electrode reactions in a system which consists of two electrodes, a cation-selective membrane, an anion-selective membrane, and a porous diaphragm through which a salt solution is pumped. In contrast, the electrodialysis process consists of a stack containing cation-selective membranes, bipolar membranes, anion-selective membranes, and electrodes at the end of the stack. The electrolysis unit requires a set of electrodes for each unit cell. Whereas, the stack utilized in the electrodialysis unit contains a large number of spaced apart membranes assembled between a single set of electrodes. Costs of electrodes and the electrical connections needed for electrolysis process are therefore significantly higher than for the electrodialysis process. Additionally, electrodiaytic water splitting is energetically a much more efficient process than electrolysis because it eliminates the over voltages associated with gas evolution at the electrodes.
In view thereof, the method and apparatus of the present invention utilizes an electrodialysis process to directly split a salt solution to generate acid and base solutions.
The process is conceptually a simple one and can be represented by the equation: EQU MX (Salt) + H.sub.2 O.fwdarw. HX (Acid) + MOH (Base).
To effect and maintain separation of the various species, ion exchange membranes are used. The most crucial of these is the bipolar membrane, so called because it is composed of two distinct parts which are selective to ions of opposite charge. Under the influence of an applied direct current, such a sandwich membrane is capable of forcibly dissociating water to form equivalent amounts of hydrogen and hydroxyl ions. When used in conjunction with other cation-selective and anion selective (monopolar) membranes, the assembly constitutes an economical water splitting apparatus that generates acid and base solutions.
Literature on the use of bipolar membrane water splitting to generate acid and base solutions dates back at lease to the mid-fifties. A good discussion of this process is presented in a paper entitled Use of Bipolar Membranes for Generation of Acid and Base--an Engineering and Economic Analysis by K. Nagasubramanian, F. P. Chlanda, and Kang-Jen Liu, Journal of Membrane Science, 2 (1977) 109-124, and in U.S. Pat. Nos. 3,705,846, 4024,043, 4,082,835, 4,219,396 and 4,3ll,77l, the discussion in which paper and patents are hereby incorporated by reference.